I've ridden my fixed pretty much exclusively for the last couple of years (total about 8k miles so far).
as I live in suffolk there's no really major hills. the ones that are here seem to have been worn down in the last couple of years as they dont seem anywhere near as steep now!
When I ride fixed for commuting, I ride because of the low maintenance.
When I ride fixed for training, I ride because of the presumed benefits of the extra effort (not sure f there is any benefit, but it seems that way!)
When I ride fixed for leisure, I ride because of the simplicity - it's very enjoyable and care-free.
Having said that, I have just started to build a new geared bike so that I can go faster and longer when I eventually join the local cycling club!